The invention relates to a control system for an at least temporarily four wheel-driven motor vehicle.
Such control systems are described in German Patent Document No. DE 100 54 023 A1, for example, according to which a torque distribution device is known for modifying the torque distribution ratio between the wheels on the front axle and the wheels on the rear axle by corresponding control of a friction clutch as a longitudinal lock (transmission clutch). The driving characteristics of a vehicle may be influenced considerably by determining a torque distribution ratio. DE 100 54 023 A1 relates in particular to driving dynamics when cornering.
In the following description, as a generalization the wheels permanently connected to the drive unit are referred to as primary drive wheels, and the wheels which may be connected as required to the drive unit via the transmission clutch are referred to as secondary drive wheels.
Exemplary embodiments of the present invention provide an improved control system of the aforementioned type with respect to the accuracy of control of the transmission clutch.
The invention is based on the finding that for a new system, the characteristic curve in the control unit by which a displacement path is assigned to a setpoint clutch torque results in a relatively accurate actual clutch torque corresponding to the setpoint clutch torque. In an earlier system, however, the same displacement path could result in an actual clutch torque that was less than the setpoint clutch torque.
This can be identified when a difference between the setpoint clutch torque and the clutch torque that is actually set can be determined. The setpoint clutch torque is known, but not the clutch torque that is actually set. The clutch torque that is actually set is computed in the simplest manner possible according to the invention, without the use of additional sensors. The computation is performed only for an operating condition, in particular for stably operating wheels, for which simplifications may be made in the computations, since certain unknown variables may be disregarded.
When reviewing the known slip curves for the front axle and the rear axle, the linear region of the slip curve should preferably be present to ensure stable operation of all wheels without slip. In other words, the tire longitudinal rigidities (k) should be the same for the wheels on the front axle and on the rear axle to ensure that the tire response on the wheels on the front axle are as similar as possible to those on the rear axle (also see the illustration of known slip curves in FIGS. 3a and 3b).
The monitoring of the control accuracy according to the invention is performed preferably by use of the wheel speed sensors or wheel angular velocity sensors, which are present anyway. In this regard, the accuracy of the diagnosis can be increased when it is made under additional operating conditions which ensure that the least possible wheel slip can occur, except as provided by the control inaccuracy of the transmission clutch or the actuator thereof.
The advantage of the invention lies in the diagnosis of control accuracy without additional sensors.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.